In the manufacture of ever more miniaturized, complex and compact electronic circuits; the art has developed to hybrid integrated circuits in which many conventional integrated circuit chips are mounted on substrates much like resistors were previously mounted on printed wiring boards. One of the more promising techniques for utilizing these hybrid integrated circuits involves the use of a ceramic substrate with gold conductors defining a plurality of bond sites to which integrated circuits are bonded.
The integrated circuits or chips; which are typically thin dies of silicon on which entire, multi-transistor circuits are formed; are typically on the order of 2mm by 2mm squares less than a millimeter thick. The areas on the chip to which connections are made to the external circuit are attached to cantilevered gold beams which extend outwardly from the edges of the chip. These chips are either painted with magnetic ink and mounted in rows and columns in an orthogonal array on a magnetic carrier, or they can be applied to an adhesive carrier. The mounting of a chip on the carrier can be accomplished with moderate accuracy as to X and Y coordinates as well as to rotation (.theta.). But the chips cannot be positioned on the carrier with the degree of accuracy required for pick-up by a bonding head and transfer to a bond site on the substrate, without some additional fine positioning.
Manual fine positioning is now typically done by an operator peering through a microscope at an optical beam splitter to superimpose an image of the beam-leaded chip upon an image of the welding or bonding tool which is also the chip pick-up tool. When these images are aligned, the optical beam splitter is retracted and the bonding tool is lowered. By means of a vacuum, the tool picks the chip from the carrier. The bond site on the substrate is then positioned under the bonding tool and chip using the same optical beam splitter. When alignment is assured, the tool descends and bonds the beam leads of the chip to suitable gold pads at the bond site on the ceramic substrate.
This manual technique of positioning the chip and the bond site is tedious, exacting, and expensive. Semi-automatic techniques have been developed in which the carrier is automatically indexed by a digital-controlled machine to position the next succeeding chip within the field of the operator's microscope. The same type of digital-controlled machine can be place next suitable bond site under the operator's microscope for bonding. This type of semi-automatic machine is of great assistance to the operator as the human operator need only perform the critical operation of fine positioning of both the chip and the substrate. These digital-controlled operations are direct analogies to those which have been used for the past decade or two in the field of automated machine tools.
Further efforts have been made to reduce the cost and human error which are part of the manual fine positioning of chips and bond sites. Several systems have been deviced to sense the location of the beam leads of the chip and also the pads of the bond site. These systems typically generate an error signal which is then used for fine correction of the position of the chip or the bond site.
One such system has been marketed by Precision Equipment Company, Inc., a subsidiary of Kulick and Soffa, Inc. and involves viewing the substrate or the bond site with a TV camera and projecting a magnified image of the chip or bond site onto the vidicon target of the TV camera. As opposed to the orthogonal raster of the conventional TV picture, a circular scan of fixed diameter is moved about the target; and the resultant video signal is compared with a stored representation of the video signal derived previously from a properly positioned chip or bond site. When a match is noted between the two signals, an error signal in three dimensions is generated; and the location of the bond site or chip is corrected in the X, Y, and .theta. dimensions.
It is also known that a typical raster scan of the vidicon target can be similarly compared with a stored representation of a properly positioned chip for generating error signals. The transition from dark to light image resulting from the scanning of the video beam as it crosses an edge of a gold reflection can be used to determine the locations of the highly reflective gold beam leads of the chip or the highly reflective bonding pads of the substrate.